The present invention relates to a diaphragm for use in a cell for the electrolysis of an aqueous solution of an alkali metal halide.
In the known diaphragm cells, in particular the cells for the electrolysis of a sodium chloride brine, the diaphragm is generally made of a layer or a sheet of asbestos, laid on a foraminous structure of iron, which forms the cathode of the cell.
The variety of asbestos generally used is chrysotile, because this gives the diaphragm good cohesion and, consequently, good mechanical resistance.
The known chrysotile diaphragms nevertheless have the disadvantage of dimensional instability with the passage of time. At the start of electrolysis these known diaphragms tend to swell, this swelling being followed by progressive consolidation under the effect of the hydrostatic pressure ruling in the cell. These variations with the passage of time in the volume and shape of the diaphragm are unfavorable to the electrolysis. Furthermore, they have a detrimental influence on the geometry chosen for the cell. In particular, the swelling of the diaphragm at the start of its life necessitates the provision in the cell of anodecathode separations distinctly greater than the optimum value for a cell in a normal condition.
Chrysotile diaphragms also have thhe disadvantage of rapid deterioration in contact with acid electrolytes.
In order to overcome this last disadvantage, it has been proposed in U.S. Pat. No. 3,505,200 to make diaphragms of anthophyllite alone or in admixture with chrysotile. Although the anthophyllite variety of asbestos has good resistance to acids, nevertheless diaphragms of pure anthophyllite are difficult to use in electrolytic cells because of their lack of cohesion. The addition of chrysotile to the anthophyllite substantially improves the cohesion of the diaphragm, but to the detriment of its resistance to acids.
These known diaphragms based on anthophyllite also suffer from the disadvantage, already mentioned above for chrysotile diaphragms, in that they change in volume and shape in the course of electrolysis.
In order to increase the cohesion of asbestos diaphragms and to improve their stability as to shape and dimensions, it has been proposed, in West German Patent Application No. 2,140,714, to coat the asbestos fibers of the diaphragm with a binding agent containing a fluorinated resin. In these known diaphragms, the presence of the resin, dispersed between the asbestos fibers, has however the highly disadvantageous effect of reducing significantly the permeability of the diaphragm. Thus, in this known diaphragm an acceptable cohesion can be obtained only with sacrifice of permeability or, conversely, an acceptable permeability of the diaphragm can be obtained only at the expense of the cohesion and the stability of shape and dimensions of the diaphragm.